There are two possibilities, the first is that the Defenders attack first and the second possibility is that the "Dead" are removed from their respective parties after the completion of the round, meaning after every npc/player in both parties have attacked. I believe the second possibility is most likely.

Remember the Combine is delayed real-time and it is designed to allow enough time for the defender to react when they are initially off-line so this feature is necessary. Otherwise the defender will be at a severe disadvantage, more than they already have.

Asume you and the enemy are standing across from each other old British style, and all of you pull the trigger at the same moment. All the attacks happen, but bullets can hit things even after they are dead. This is not the dnd turn based or initiative based system, this is a delayed RTS(of sorts) where everything can shoot at once.